1. Field of the Invention
This invention relates to switchgear used in electric power distribution systems, and particularly to a switchgear assembly housing a three-phase tie or main circuit breaker and to the arrangement for connecting the circuit breaker to upper and lower horizontal buses in the switchgear assembly.
2. Background Information
Switchgear assemblies typically include a series of metal cabinets or sections housing electrical apparatus and associated conductors for connecting the electrical apparatus in an electric power distribution system. The electrical apparatus can include electrical switches such as circuit breakers and network protectors which provide protection, disconnect switches for isolating parts of the electric power distribution system and transfer switches for transferring between alternate power sources. The electrical apparatus can also include measuring and instrumentation equipment. Switchgear operating at voltages up through 600 volts are categorized as low voltage switchgear.
Often low voltage switchgear assemblies are configured with substation transformers connected on both ends of the switchgear assembly through main breakers. The purpose of this configuration is to provide multiple power sources to the switchgear in the event one of the sources fails. A tie breaker is placed in the center of the line-up to provide the transition of power from one source to the other. The main and tie circuit breakers are often interlocked either electrically or mechanically such that both mains and the tie breaker cannot be closed simultaneously. Thus, one of two operating conditions exist: 1) both main circuit breakers are closed and the tie circuit breaker is open, thus the switchgear assembly is fed via two sources, or 2) one main circuit breaker is closed, the other main circuit breaker is open and the tie circuit breaker is closed so that the entire switchgear assembly is fed from one source.
A typical section of a switchgear assembly is equipped to provide four vertically stacked positions for circuit breaker or instrumentation mounting. The top position is labeled "A", second from the top is labeled "B", third from top is "C" and the bottom is defined as "D". In existing switchgear assemblies of the assignee of this invention, the placement of tie circuit breakers is exclusively in the B position. The tie circuit breaker in this arrangement is fed strictly through the vertical bus via horizontal main buses on both sides. To accommodate this arrangement, the vertical bus or risers is split with the upper and lower sections connected to separate horizontal buses to force the primary current path through the tie circuit breaker. Thus, the upper main horizontal bus is connected to the upper disconnects of the tie circuit breaker while a lower main horizontal bus is connected to the lower disconnects. Due to the placement of the main bus taps (the connectors from the vertical bus to the horizontal main bus) the B location provides the logical position for the vertical bus split. In this position, the upper main horizontal bus is above the stabs which connect the upper section of the vertical bus to the upper disconnects on the tie circuit breaker, and the lower main horizontal bus is below the stabs which connect the lower sections of the vertical bus to the lower disconnects.
While the most common arrangement for main circuit breakers in a low voltage switchgear assembly is to connect the upper disconnects to the utility lines through cabling connected to runbacks and to connect the load side to a vertical bus which extends the full height of the cabinet, in the arrangement discussed above where main circuit breakers are provided on both ends of the switchgear assembly often the main circuit breakers are each mounted in the "B" position and connected to the upper and lower sections of an associated split vertical bus.
Placing the main or tie circuit breakers in the "B" position has some limitations. First, depending upon the number of feeder circuits between the main and the tie circuit breakers, an additional switchgear section might be needed just to accommodate the required feeder circuits so that an equal number of circuits are connected on both sides of the tie circuit breaker. Secondly, associated metering and control devices are typically mounted above the circuit breakers. Placing the circuit breakers in the "B" position only leaves the "A" position for this purpose. This is especially a problem for the main circuit breakers which usually require more metering than the other circuit breakers.
There is a need, therefore, for improved switchgear assemblies, and particularly to an improved arrangement in switchgear assemblies for mounting tie circuit breakers and main circuit breakers.